1. Technical Field
This invention relates to casting one or more layers of a film-forming liquid on a substrate, and to the apparatus employed for that purpose.
2. State of the Art
The prior art is replete with means for casting a film-forming liquid upon a moving substrate. In a known coating device of this type, (U.S. Pat. No. 4,335,672), a suction device is provided with fixed walls, a low pressure chamber, and a vacuum chamber, having a given set volume. The two chambers are connected with each other via the gap between the suction device and the substrate. Low pressure in the low pressure chamber stabilizes the bead which is formed in the casting gap by the casting material. Since the low pressure chamber is surrounded on three sides by the vacuum chamber, the disturbances produced by the incoming air from the environment are extensively avoided. As a result, the bead is guaranteed not to be subjected to any disturbances from air turbulence or pressure fluctuations so that a uniform application of the cast layer is obtained. Periodic disturbances of the cast layer are at times observed, however, based on pressure fluctuations in the low pressure chamber. Certain problems exist, moreover, when changes in the operating conditions (different sheet speed, a different thickness of the substrate, etc), requires reestablishing the optimum relationships, even when the suction device is radially adjusted to change the gap between the suction device and the substrate.
The object of the present invention is to provide a device of the above described type which permits further reductions in pressure fluctuations and turbulence in the low pressure chamber when the operating conditions are varied. In a first embodiment of the invention this objective is met by adjusting the partition and/or the end wall in circumferential direction.
According to a second embodiment of the invention which can be used simultaneously with the first embodiment, this objective is met by connecting the low pressure chamber and the vacuum chamber by means of a bypass.